The present invention relates to a process for arc fusion welding under atmospheric conditions a nitrogen alloyed steel, in which the bulk share of nitrogen in the steel is above its solubility limit under atmospheric conditions.
This type of steel can be produced, e.g., as Cr-Mn steel, by the so-called pressure electroslag remelting process (DESU process, see, e.g., DE-PS 29 24 415) or in a plasma arc furnace. The high nitrogen content is supposed to bring about a clear increase in the hardness level, without having any negative influence on the toughness and molding properties.
The solubility limit, also called simply solubility, of a steel for nitrogen essentially depends on the partial pressure of the nitrogen, and on the nature and amount of alloy elements in the steel, whereby the partial pressure of the nitrogen refers to the partial pressure of the nitrogen during the fusible condition of the steel. The solubility of the steel for nitrogen increases with the square root of this partial pressure.
While the nitrogen solubility of pure iron [% N.sub.g ].sub.Fe with a partial nitrogen pressure of 0.8 bar is 0.0395% (the % numbers in these documents always refer to % by bulk or weight), the nitrogen solubility limit N.sub.g (hereafter referred to as nitrogen solubility for short) of an iron-rich multicomponenet alloy, thus of a steel, is EQU [% N.sub.g ].sub.Fe,C,X,Y, . . . =0.0395/f.sub.N ( 1)
whereby f.sub.N is the so-called activity factor which is the power formed, according to Frohberg, Martin G.: Thermodynamik fuer Metallurgen und Werkstofftechniker, Leipzig: VEB Deutscher Verlag fuer Grundstoffindustrie 1980, from base 10 and the exponents consisting of the sequence EQU e.sub.N.sup.C [% C]+e.sub.N.sup.X [% X]+e.sub.N.sup.Y [Y]+ . . .
In other terms, the regular or decimal logarithm of the so-called activity factor f.sub.N is equal to the above sequence, whereby EQU e.sub.N.sup.C, e.sub.N.sup.X, e.sub.N.sup.Y, . . .
represent the so-called interaction coefficient of carbon and the other alloy elements (here designated generally by X, Y, . . . ) for nitrogen and EQU [% X], [% Y], . . .
represent the share of the respective alloy elements in the steel.
According to H. Schenck, M. G. Frohberg and H. Graf: Untersuchungen ueber die Beeinflussung der Gleichgewichte von Stickstoff mit fluessigen Eisenlegierungen durch den Zusatz weiterer Elemente (II) [Investigations Concerning the Influence on the Equilibria of Nitrogen with Liquid Iron Alloys by the Addition of Other Elements], Archiv fuer das Eisenhuettenwesen 30 [Archives for the Metallurgical Industry] (1959) 9, pages 533-537, the interaction coefficients for the individual alloy elements are as follows:
______________________________________ Element e.sub.N Element e.sub.N ______________________________________ chromium -0.045 niobium -0.061 carbon +0.125 silicon +0.065 manganese -0.020 titanium -0.530 molybdenum -0.013 vanadium -0.010 nickel +0.010 tungsten -0.0015 ______________________________________
Based on these values it is possible to determine the nitrogen solubility N.sub.g for a nitrogen partial pressure of 0.8 bar for each steel for which the chemical composition is known, as: ##EQU1##
For the arc fusion welding of highly nitrogenized steels in normal shops and construction sites where atmospheric conditions are always present, the nitrogen is subject to only a low partial pressure of about 0.8 bar. Forcefully dissolved nitrogen, i.e. nitrogen which, during production, was added beyond the solubility of the steel, escapes from the molten areas of the steel, which leads to a high degree of porosity in the welding zone. The area of application for pressure nitrogenized steels, e.g., austenitic, non-magnetic steels with high strength, thus, is limited to construction parts or work pieces which do not have to be welded, as this type of steel is not considered weldable with this behavior.